Note: This thread references this paper by Jones, Underwood and Sleep (referred to as JUS) that defines inner and outer "reaches" for jovians. This defines the band around a jovian within which a rocky planet will be ejected from the system within 1000 My, due to the jovian's gravitational influence. I'm also assuming that if a potential planet is located between a jovian's 4:1 and 3:2 distances then its formation is disrupted by the jovian's gravity and it forms an asteroid belt instead.

I noticed something interesting about Jupiter - its 3:2 distance is at 3.97 AU, but its Inner Reach distance (determined using the formula in JUS) is 3.88 AU - these regions therefore overlap, which may explain this graph from the New System thread - there's just no orbit beyond the 3:2 where asteroid orbits can be stable:

Now, here's one of the systems I generated (the one from the New System thread):

The planet in orbit 4 (2.291 AU) is a Jupiter-class gas giant, between Jupiter and Saturn in size. The rocky planet that was in Orbit 3 (0.969 AU) is an asteroid belt because its semimajor axis is between the 4:1 and 2:1 orbits of the jovian in orbit 4. However, this jovian's Inner Reach extends to 1.882 AU, but it's 3:2 orbit (not shown on the table) is at 1.75 AU - thus there's a gap between the Inner Reach and the 3:2 resonance.

One thing to point out is that the values used to determined the JUS Reaches are determined experimentally - basically they ran a souped up version of Gravity Simulator and found out where a planet with a mass equal to the Earth+Moon survived for 1000 My and where it was ejected before that time. The resonance values are however determined theoretically by calculating them from the jovian's orbital period. It's possible that the JUS values should actually correspond roughly to resonances... or maybe they're unrelated.

So the question is... are the orbits in this gap actually stable? To try to test this, I set up a reduced version of the 2840 system in gravsim, with just the sun and the jovian at 2.291 AU. I then put 20 asteroids between 1.62 and 1.74 AU (red, within the 3:2), 20 asteroids between 1.75 and 1.88 AU (yellow, between the 3:2 and the Inner Reach), 20 asteroids between 1.89 and 2.01 AU (green, beyond the Inner Reach distance). I hoped that the yellow ones would be most stable since they're beyond 3:2 and also beyond the Inner Reach...

I've uploaded the gsim file if you want to play around with it yourselves.

Well I can't show the animation, because it's too big... but I can show a still of the end result of SMA vs Ecc, and it's a bit surprising. It looks as if actually the asteroids between the two zones aren't actually much more stable than the ones within the resonance zone, but they definitely are more stable than the ones within the Reach zone (which are flung into wildly different orbits pretty quickly in the simulation - that's the line sloping toward the star from the 2 AU region).

So for this system at least I'd say that any planets that want to form between the 3:2 and the Inner Reach are probably going to still be disrupted by the jovian and become belts instead, but they probably won't be ejected completely from the system. The squat slab-like peak beneath that is the 3:2 resonance (skewed to the left here, for reasons unclear to me).

I'm not sure. I think it's to do with the angle that the 3:2 peak is skewed at, but maybe it's something different. It doesn't seem to correspond to a resonance itself though - here are all the resonances for the jovian that I can think of: